The present invention relates to a magnetic recording medium having excellent electromagnetic transfer characteristics and comprising a plurality of layers, i.e. at least two layers including an upper magnetic layer where ferromagnetic fine powder and a binder are dispersed and a lower coating layer.
The magnetic recording medium is widely used in tape for tape recording, video tape, floppy disk, etc. In the magnetic recording medium, a magnetic layer where ferromagnetic powder is dispersed in a binder is deposited on a non-magnetic support member.
The magnetic recording medium must be at high level in various characteristics such as electromagnetic transfer characteristics, running durability and running performance. Namely, an audio tape for recording and reproducing musical sound must have higher ability to reproduce original sound. A video tape must have excellent electromagnetic transfer characteristics such as high ability to reproduce original image. In particular, a magnetic recording medium for digital video tape recorder, which must be suitable for high density recording, must have higher electromagnetic transfer characteristics.
In addition to the excellent electromagnetic characteristics as described above, the magnetic recording medium must have good running durability as already described. To ensure good running durability, abrasive and lubricant are added in the magnetic layer.
However, to obtain good running durability by the use of abrasive, it is necessary to increase the quantity of the added abrasive to some extent. As a result, filling degree of the ferromagnetic powder decreases. In case an abrasive material having larger particle size is used to obtain good running durability, the abrasive material is more likely to protrude or to produce roughness on the surface of the magnetic layer. Therefore, the improvement of running durability by the use of abrasive often leads to deterioration of the electromagnetic transfer characteristics.
When it is tried to improve the running durability by the use of lubricant, it is necessary to increase the quantity of the added lubricant. As a result, the binder is more likely to be plasticized, and the durability of the magnetic layer tends to decrease.
Also, to improve the durability and the electromagnetic transfer characteristics, the binder, serving as a major component of the magnetic layer, also plays an important role. The materials used in the past such as vinyl chloride resin, cellulose resin, urethane resin, acrylic resin, etc. are low in anti-abrasive property of the magnetic layer, and this often results in contamination of running components of the magnetic tape.
To overcome the above problems, a method to use hard binder and to increase hardness of the magnetic layer is adopted.
Further, to improve the durability and the electromagnetic transfer characteristics, it has been proposed to provide at least two layers including an upper magnetic layer and a lower coating layer. For example, the use of aromatic polyurethane containing a structure of ethylene oxide or propylene oxide of bisphenol A in the magnetic layer other than the upper layer is disclosed in JP(A) 07176042. However, dispersion property of the lower layer is low, and both electromagnetic transfer characteristics and durability are not sufficient. Also, it is described that Tg=-50.degree. C. to +50.degree. C. In the embodiment, polyurethane and butanediol adipate polyol are used which use polyethylene oxide addition product of bisphenol A as chain extender, but Tg=-20.degree. C., and strength of the entire coating film is decreased, resulting in lower durability.
The use of polyvalent OH urethane in the lower non-magnetic layer is disclosed in JP(A) 04238111. In the embodiment, a compound containing caprolactone polyol by 60% is used as a long-chain diol, but it is insufficient in dispersion property.
In JP(A) 02113424 and JP(A) 05242450, the use of urethane containing negative functional groups in an intermediate layer and a non-magnetic layer is disclosed. Although there is no description on the content of long-chain diol in polyurethane, polyester type compounds are used in the embodiment, while these are low in dispersion property and durability.
In JP(A) 03083221 (U.S. Pat. No. 5,258,223), the use of polyester urethane of Tg=-20 to +40.degree. C. in an upper layer and the use of polyester urethane of Tg=-50.degree. C. to -10.degree. C. in a lower layer are disclosed. Because the value of Tg is low, the strength of coating film decreases, and durability is not satisfactory.
Further, in JP(A) 03088119 (EP 415,708), a magnetic recording medium is described, which uses aliphatic polyurethane in a lower layer and aromatic polyurethane in an upper layer. However, there is no description on the contents of long-chain diol, short-chain diol and ether. In the embodiment, it is described that Tg.ltoreq.20.degree. C. but strength of coating film is low, and durability and dispersion property are not sufficient.
It is an object of the present invention to provide a magnetic recording medium, which has excellent dispersion property, high electromagnetic transfer characteristics, and high strength coating film, and hence exhibits excellent repeated running property and still durability.